High clarity carboxylated nitrile rubber

ABSTRACT

The present invention relates to a high clarity water-sensitive carboxylated nitrile rubber composition which is particularly useful in making photopolymer formulations for printing plates utilized in flexographic printing applications. The present invention more specifically discloses a high clarity water-sensitive carboxylated nitrile rubber composition which is comprised of (a) a carboxylated nitrile rubber which is comprised of repeat units which are derived from about 33 weight percent to about 77 weight percent 1,3-butadiene, from about 15 weight percent to about 45 weight percent acrylonitrile and from about 8 weight percent to about 22 weight percent methacrylic acid and (b) from about 1 to about 10 phr of at least one organic acid which contains at least three carboxyl groups. The carboxylated nitrile rubber of this invention will typically have a Mooney ML-4 viscosity of less than about 80 and a volume swell of greater than 40 percent. It will also have sufficient clarity to allow print having 6-point font to be read through a 0.125-inch thick sheet of said carboxylated nitrile rubber. In applications where improved scorch safety is required, the rubber composition will also contain from about 0.1 phr to about 7 phr of at least one oligomerized fatty acid.

This application claims benefit of U.S. Provisional Application Ser. No. 60/008,738 filed Dec. 13, 1995 and is a continuation-in-part-application of U.S. patent application Ser. No. 08/766,449, filed on Dec. 12, 1996 U.S. Pat. No. 5,990,215.

BACKGROUND OF THE INVENTION

It is important for photopolymer compositions utilized in making printing plates to have a high degree of clarity so that ultra-violet light which is transmitted through a photographic negative can penetrate the composition and initiate cure. It is also critical for the photopolymer composition to exhibit a high degree of water swell which allows for easy scrub-off of that part of the printing plate which was not cured. There are also certain mechanical requirements needed from such photopolymer compositions.

U.S. Pat. No. 4,045,231 discloses the utilization of polystyrene-polybutadiene-polystyrene triblock polymers in printing plates. U.S. Pat. No. 4,162,919 discloses the employment of styrene-isoprene diblock polymers and styrene-isoprene diblock polymers in flexographic printing plates. U.S. Pat. No. 5,344,743 discloses flexographic photosensitive printing plates which are made with formulations comprising triblock polymers of poly(vinylpyridine)-poly(butadiene) or poly(isoprene)-poly(vinylpyridine).

Certain carboxylated nitrile rubbers, such as Chemigum® NX775 carboxylated nitrile rubber which is sold by The Goodyear Tire & Rubber Company, offer most of the physical requirements needed from a base polymer for printing plate photopolymer compositions. However, such carboxylated nitrile rubbers do not have adequate clarity and are too water-resistant for utilization in such applications. It is also important for carboxylated nitrile rubbers utilized in such applications to have a Mooney ML-4 viscosity of less than about 80.

U.S. Pat. Nos. 4,415,690, 4,435,535 and U.S. Pat. No. 4,452,936 disclose techniques for preparing carboxylated nitrile rubbers which are not suitable for utilization in making printing plate photopolymer compositions because of their inadequate clarity and water swell characteristics. U.S. Pat. No. 4,415,690 discloses the utilization of succinic anhydride derivatives as scorch inhibiting agents. U.S. Pat. No. 4,435,535 discloses the utilization of adipic acid as a scorch inhibiting agent and U.S. Pat. No. 4,452,936 discloses the utilization of oligomerized fatty acids as scorch inhibiting agents.

SUMMARY OF THE INVENTION

It has been determined that carboxylated nitrile rubber which is suitable for utilization as a base polymer for printing plate photopolymer compositions can be made by utilizing the process of this invention. This process differs from the process disclosed in U.S. Pat. No. 4,452,936 in several critical respects. For instance, the level of methacrylic acid incorporated into the carboxylated nitrile rubber is increased so as to be within the range of about 10 weight percent to about 20 weight percent. It is also critical for the coagulation process carried out in recovering the carboxylated nitrile rubber from the latex to be accomplished without utilizing polyelectrolytes, such as Nalco™ 108 polyelectrolyte, as coagulation aids. The utilization of at least one organic acid which contains at least three carboxyl groups, such as citric acid, in the carboxylated nitrile rubbers of this invention is also critical. The organic acid can be added to the latex of the carboxylated nitrile rubber before coagulation or it can be added to the dry rubber after coagulation.

The present invention more specifically discloses a high clarity water-sensitive carboxylated nitrile rubber which is comprised of repeat units which are derived from about 33 weight percent to about 77 weight percent 1,3-butadiene, from about 15 weight percent to about 45 weight percent acrylonitrile and from about 8 weight percent to about 22 weight percent methacrylic acid. Generally, from about 1 to about 10 phr of at least one organic acid which contains at least three carboxyl groups is included in the carboxylated nitrile rubber to keep the Mooney ML-4 viscosity of the rubber in the desired range. The carboxylated nitrile rubber of this invention will typically have a Mooney ML-4 viscosity of less than about 80 and a volume swell of greater than 40 percent. In applications where scorch resistance is required, the carboxylated nitrile rubber composition will also contain from about 0.1 phr to about 7 phr of at least one oligomerized fatty acid.

The present invention further reveals a process for preparing a high clarity water-sensitive carboxylated nitrile rubber composition which comprises the steps of (1) free radical polymerizing from about 30 weight percent to about 73 weight percent 1,3-butadiene, from about 20 weight percent to about 50 weight percent acrylonitrile and from about 7 weight percent to about 20 weight percent methacrylic acid in an aqueous polymerization medium in the presence of at least one emulsifier to produce a carboxylated nitrile rubber latex; (2) adjusting the pH of the carboxylated nitrile rubber latex to above 4 by the addition of a base to produce a pH-adjusted carboxylated nitrile rubber latex; (3) coagulating the pH-adjusted carboxylated nitrile rubber latex by the addition of a combination of coagulating agents which consists essentially of at least one strong inorganic acid, calcium chloride and at least one organic acid which contains at least three carboxyl groups, to separate wet carboxylated nitrile rubber composition from an aqueous phase; and (4) drying the wet carboxylated nitrile rubber composition to produce said high clarity water-sensitive carboxylated nitrile rubber composition.

The organic acid is used to keep the Mooney ML-4 viscosity of the carboxylated nitrile rubber composition below about 80. The Mooney ML-4 viscosity of the carboxylated nitrile rubber composition will typically be within the range of about 30 to about 70. The Mooney ML-4 viscosity of the carboxylated nitrile rubber composition will preferably be within the range of about 40 to about 60. A saponified oligomerized fatty acid can optionally be added to the pH-adjusted carboxylated nitrile rubber latex prior to coagulation to improve the scorch resistance of the high clarity carboxylated nitrile rubber.

DETAILED DESCRIPTION OF THE INVENTION

Carboxylated nitrile rubbers contain repeat units which are derived from 1,3-butadiene, acrylonitrile and methacrylic acid. They can be synthesized by the free radical terpolymerization of the 1,3-butadiene, acrylonitrile and methacrylic acid monomers under emulsion polymerization conditions. Such emulsion polymerizations generally utilize a charge composition which is comprised of water, the monomers, an initiator and an emulsifier (soap). The emulsions polymerizations of this invention can be conducted over a very wide temperature range from about 0° C. to as high as about 100° C. It is normally preferred for the emulsion polymerizations of this invention to be carried out at a temperature which is within the range of about 5° C. to about 60° C. It is generally more preferred for the emulsion polymerization to be conducted at a temperature which is within the range of about 15° C. to about 30° C.

The ratio of monomers utilized in the charge composition can vary. However, the monomer charge composition will normally contain from about 30 to about 73 weight percent 1,3-butadiene, from about 20 to about 50 weight percent acrylonitrile and from about 7 to about 20 weight percent methacrylic acid. The monomer charge composition will more typically contain from about 40 to about 52 weight percent 1,3-butadiene, from about 36 to about 44 weight percent acrylonitrile and from about 12 to about 16 weight percent methacrylic acid. The monomer charge composition will preferably contain from about 43 to about 49 weight percent 1,3-butadiene, from about 38 to about 42 weight percent acrylonitrile and from about 13 to about 15 weight percent methacrylic acid.

The charge composition used in the preparation of the latices of this invention will contain a substantial quantity of water. The ratio between the total amount of monomers present in the charge composition and water can range between about 0.2:1 and about 1.2:1. It is generally preferred for the ratio of monomers to water in the charge composition to be within the range of about 0.8:1 and about 1.1:1. For instance, it is very satisfactory to utilize a ratio of monomers to water in the charge composition of about 1:1.

The charge composition will also normally contain from about 0.5 phm (parts per hundred parts by weight of monomer) to about 6 phm of at least emulsifier. It is normally preferred for the emulsifier to be present in the polymerization medium at a level within the range of about 1 phm to about 5 phm. It is generally more preferred for the charge composition to contain from about 2 to about 4 phm of the emulsifier.

The emulsifiers used in the polymerization of this invention may be charged at the outset of the polymerization or may be added incrementally or by proportioning as the reaction proceeds. Generally, anionic emulsifier systems provide good results; however, any of the general types of anionic, cationic or nonionic emulsifiers may be employed in the polymerization.

Among the anionic emulsifiers that can be employed in the emulsion polymerizations of this invention are fatty acids and their alkali metal soaps; such as, caprylic acid, capric acid, pelargonic acid, lauric acid, undecylic acid, myristic acid, palmitic acid, margaric acid, stearic acid, arachidic acid, and the like; amine soaps of fatty acids such as those formed from ammonia, mono- and dialkyl amines, substituted hydrazines, guanidine and various low molecular weight diamines; chain-substituted derivatives of fatty acids such as those having alkyl substituents; napthenic acids and their soaps and the like; sulfuric esters and their salts, such as the tallow alcohol sulfates, coconut alcohol sulfates, fatty alcohol sulfates, such as oleyl sulfate, sodium lauryl sulfate and the like; sterol sulfates; sulfates of alkylcyclohexanols, sulfation products of lower polymers of ethylene as C₁₀ to C₂₀ straight chain olefins and other hydrocarbon mixtures, sulfuric esters of aliphatic and aromatic alcohols having intermediate linkages, such as ether, ester or amide groups such as alkylbenzyl (polyethyleneoxy) alcohols, the sodium salt of tridecyl ether sulfate; alkane sulfonates, esters and salts, such as alkylchlorosulfonates with the general formula RSO₂ Cl, wherein R is an alkyl group having from 10 to 20 carbon atoms and alkylsulfonates with the general formula RSC₂ --OH, wherein R is an alkyl group having from 1 to 20 carbon atoms; sulfonates with intermediate linkages such as ester and ester-linked sulfonates such as those having the formula RCOOC₂ H₄ SO₃ H and ROOC--CH₂ --SO₃ H, wherein R is an alkyl group having from 1 to 20 carbon atoms such as dialkyl sulfosuccinates; ester salts with the general formula: ##STR1## wherein R is an alkyl group having from 1 to 20 carbon atoms; alkaryl sulfonates in which the alkyl groups contain preferably from 10 to 20 carbon atoms; e.g., dodecylbenzenesulfonates, such as sodium dodecylbenzenesulfonates; alkyl phenol sulfonates; sulfonic acids and their salts such as acids with the formula RSO₃ Na, wherein R is an alkyl and the like; sulfonamides; sulfamido methylenesulfonic acids; rosin acids and their soaps; sulfonates derivatives of rosin and rosin oil; and lignin sulfonates and the like.

Rosin acid soap has been used with good success at a concentration of about 5 percent by weight in the initial charge composition used in the synthesis of carboxylated elastomers. Of rosin acids, about 90 percent are isometric with abietic acid and the other 10 percent is a mixture of dehydro abietic acid and dihydro abietic acid.

The polymerization of this invention may be initiated using free radical catalysts, ultraviolet light or radiation. To ensure a satisfactory polymerization rate, uniformity and a controllable polymerization, free radical initiators are generally used. Free radical initiators which are commonly used include the various peroxygen compounds such as potassium persulfate, ammonium persulfate, benzoyl peroxide, hydrogen peroxide, di-t-butyl peroxide, dicumyl peroxide, 2,4-dichlorobenzyl peroxide, decanoyl peroxide, lauroyl peroxide, cumene hydroperoxide, p-menthane hydroperoxide, t-butylhydroperoxide, acetyl acetone peroxide, methyl ethyl ketone peroxide, succinic acid peroxide, dicetyl peroxydicarbonate, t-butyl peroxyacetate, t-butyl peroxymaleic acid, t-butyl peroxybenzoate, acetyl cyclohexyl sulfonyl peroxide and the like; the various azo compounds such as 2-t-butylazo-2-cyanopropane, dimethyl azodiisobutyrate, azodiisobutyronitrile, 2-t-butylazo-1-cyanocyclohexane, 1-t-amylazo-1-cyanocyclohexane and the like; the various alkyl perketals, such as 2,2-bis-(t-butylperoxy)butane, ethyl 3,3-bis(t-butylperoxy)butyrate, 1,1-di-(t-butylperoxy) cyclohexane and the like. Cumene hydroperoxide is a highly preferred initiator.

After the desired degree of monomer conversion has been attained, a conventional shortstopping agent, such as hydroquinone, can be added to the polymerization medium to end the polymerization. The polymerization will typically be allowed to continue until a high level conversion has been achieved. In most cases, the monomer conversion reached will be at least about 75 percent with monomer conversions of at least about 80 percent being preferred.

After the polymerization has been completed, the latex will typically have a pH which is within the range of about 3 to about 4. At pH values of less than about 4, saponified oligomerized fatty acids are only marginally soluble in latex. At lower pH levels, saponified oligomerized fatty acids are converted to free acids which are insoluble in water. Accordingly, the pH of the latex will typically be adjusted to a level above about 4 by the addition of a base, such as ammonium hydroxide, sodium hydroxide or potassium hydroxide.

After the pH of the latex has been increased to a level of greater than 4, one or more saponified oligomerized fatty acids can optionally be added to the latex to improve the scorch resistance of the rubber. Measures should be taken to insure that the saponified oligomerized fatty acids are well distributed throughout the latex. The saponified oligomerized fatty acids can be added to the latex just before coagulation if sufficient time is allowed for proper mixing to result.

In most cases, from about 0.1 phr (parts per hundred parts of rubber) to about 7 phr of saponified oligomerized fatty acids will be distributed throughout the latex. It is typically preferred to add about 0.5 phr to about 3 phr of the saponified oligomerized fatty acids to the latex. As a general rule, it is most preferred to distribute from about 0.7 phr to about 1.4 phr of the saponified oligomerized fatty acids throughout the latex.

The oligomerized fatty acids which can be used in the practice of this invention are normally oligomers of fatty acids which contain 18 carbon atoms, such as oleic acid and linoleic acid. The actual preparation, properties and structures of dimerized C₁₈ fatty acids are described in U.S. Pat. No. 2,347,562 and Cowan, John C. and Wheeler, Donald H., "Linear Superpolymers from Dilinoleic Acid," The Journal of the American Chemical Society, Vol. 66. pages 84-88 (1944), both of which are incorporated herein by reference.

Several different grades of oligomerized C₁₈ fatty acids are available commercially which vary in monomer dimer and trimer content. For example, oligomerized acids are available from Emery Industries, Inc, under the trade name Empol™. Empol 1010 contains 97 percent dimer acids and 3 percent trimer acids. Empol 1014 contains 95 percent dimer acids, 4 percent trimer acids and 1 percent of the fatty acid monomers. Empol 1016 contains 87 percent dimer acids, 13 percent trimer acids and a trace of monomer. Empol 1018 contains 83 percent trimer acids, 17 percent dimer acids and a trace of monomer. Empol 1041 contains 90 percent trimer acids and 10 percent dimer acids. Empol 1052 contains 40 percent dimer acids and 60 percent trimer and polybasic acids. Trimerized C₁₈ fatty acids have a molecular weight of about 850 contain 54 carbon atoms and have three carboxyl groups.

Any combination of oligomers of fatty acids can be utilized in practicing this invention. On a molar basis or when compared on a weight basis, trimer acids are more effective than dimer acids as scorch inhibitors. Fatty acids containing 18 carbon atoms which are dimerized and trimerized into C₃₆ and C₅₄ dimer and trimer acids will normally be used since they are readily commercially available. However, the scope of dimerized and trimerized fatty acids useful as scorch inhibitors is not limited to dimerized and trimerized C₁₈ fatty acids, since undoubtedly C₁₂ and C₂₅ fatty acids which have been dimerized and/or trimerized will also provide scorch resistance to a greater or less extent. Fatty acids containing from 12 to 25 carbon atoms which are oligomerized will be useful as scorch inhibitors for carboxylated rubbers. The oligomerization of unsaturated fatty acids containing varying numbers of carbon atoms will produce oligomers that have good properties as scorch inhibitors. For example, the codimerization of a C₁₂ fatty acid with a C₂₅ fatty acid will produce a C₃₇ dimerized fatty acid that will be a good scorch inhibitor. Any combination of any or all of the aforementioned types of oligomers of fatty acids can be used to provide excellent scorch safety when distributed throughout a carboxylated rubber.

Oligomerized fatty acids are water-insoluble materials. Thus, they must be saponified with a base to become soluble in water or in the carboxylated nitrile rubber. The oligomerized fatty acids can be readily saponified by reacting them with bases, such as aqueous potassium hydroxide, sodium hydroxide or ammonium hydroxide to yield the water-soluble salt of the oligomerized fatty acid. These salts can then be further reacted with other bases; for example, calcium chloride to yield the calcium salt of the oligomerized fatty acids.

After the saponified oligomerized fatty acids are optionally distributed throughout the latex, the carboxylated nitrile rubber can be recovered by coagulation. This coagulation will result in the conversion of the saponified oligomerized fatty acids to free acids which are insoluble in the aqueous phase but which will remain in the carboxylated nitrile rubber. The coagulation is accomplished by adding to the latex at least one strong inorganic acid, calcium chloride and optionally at least one organic acid which contains at least three carboxyl groups. The strong inorganic acid will typically be sulfuric acid and the organic acid will typically be citric acid. The calcium chloride will normally be added in an amount which is within the range of about 13 phr to about 40 phr.

The organic acid will normally be added to the latex in an amount whereby from about 1 phr to about 10 phr of the organic acid will be present in the dry carboxylated nitrile rubber after coagulation. However, the organic acid can be added in whole or in part to the dry carboxylated nitrile rubber (after coagulation) to realize the desired quantity of the organic acid in the dry rubber. For instance, the organic acid can be milled into the dry carboxylated nitrile rubber. As a general rule, it is preferred for about 1.5 phr to about 5 phr of the organic acid to be present in the carboxylated nitrile rubber. It is normally more preferred for about 2 phr to about 4 phr of the organic acid to be present in the carboxylated nitrile rubber.

After coagulation, washing can be employed to remove excess soap and/or electrolyte from the carboxylated nitrile rubber. Sometimes washing is also useful in adjusting the pH of the carboxylated elastomer that has been synthesized and recovered from the latex. After washing, the carboxylated nitrile rubber will normally be dewatered using conventional techniques. It is then dried by employing standard procedures, such as passing warm dry air through it. After being dried, it can be utilized as a base polymer in making photopolymer formulations for printing plates utilized in flexographic printing applications.

The ratio of bound monomers in the carboxylated nitrile rubber will vary from the monomer charge ratios utilized in the synthesis of the polymer due to differences in the polymerization rates of the monomers. Thus, the carboxylated nitrile rubber will typically have a different ratio of repeat units which are derived from the various monomers than was utilized in the monomer charge. In any case, the carboxylated nitrile rubbers of this invention will typically have repeat units which are derived from about 33 weight percent to about 77 weight percent 1,3-butadiene monomer, from about 15 weight percent to about 45 weight percent acrylonitrile monomer and from about 8 weight percent to about 22 weight percent methacrylic acid. The carboxylated nitrile rubbers of this invention will more typically have repeat units which are derived from about 44 weight percent to about 56 weight percent 1,3-butadiene monomer, from about 30 weight percent to about 38 weight percent acrylonitrile monomer and from about 14 weight percent to about 18 weight percent methacrylic acid. The carboxylated nitrile rubbers of this invention will preferably have repeat units which are derived from about 47 weight percent to about 53 weight percent 1,3-butadiene monomer, from about 32 weight percent to about 36 weight percent acrylonitrile monomer and from about 15 weight percent to about 17 weight percent methacrylic acid.

The carboxylated nitrile rubbers of this invention will have sufficient clarity to allow print having 6-point font to be read through a 0.125-inch thick sheet of said rubber. For instance, the rubber has sufficient clarity if the word "clear" printed in black letters on a white background using at least 6-point type can be read through a 0.125-inch thick sheet of the rubber with the unaided eye under typical indoor illumination. Said rubbers will preferably have a volume swell of greater than 60 percent and will more preferably have a volume swell of greater than 70 percent. Volume swell can be determined using the following test procedure.

% Volume Swell Test

The percent volume swell test is performed as follows:

1. Cut the polymer or composition into a piece weighing approximately 1.0-1.5 g

2. Weight sample in air (Wa₁)

3. Weight sample in water (Ww₁)

4. Place sample in the test solution for a given time and temperature. The results reported herein were for samples placed in water for 22 hours at 60° C. unless otherwise indicated.

5. Remove sample from test solution

6. Pat sample dry

7. Reweigh sample in air (Ww₂)

8. Reweigh sample in water (Ww₂)

9. Calculate % volume swell via equation 1 Wa₁ -Ww₁ =Volume 1 Wa₂ -Ww₂ =Volume 2 ##EQU1##

This invention is illustrated by the following examples which are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight.

COMPARATIVE EXAMPLE 1

In this experiment, a carboxylated nitrile rubber was synthesized and coagulated for comparative purposes. The charge composition used in the synthesis of this carboxylated nitrile rubber included 200 parts of deionized water, 0.3 parts of sodium acid phosphate, 0.1 parts of tetrasodium ethylene diamine tetraacetate, 1.0 parts of Empol® 1022 oligomerized fatty acid, 1.5 parts of dodecylbenzene sulfonic acid, 0.5 parts of the sodium salt of a condensed naphthalene sulfonic acid, 0.45 parts of a 30 percent by weight aqueous ammonium hydroxide solution, 7 parts of methacrylic acid, 0.45 parts of tertiary dodecylmercaptan, 27 parts of acrylonitrile, 0.03 parts of cumene hydroperoxide, 66 parts of 1,3-butadiene, 0.02 parts of sodium formaldehyde sulfoxylate and 0.03 parts of chelated ferrous sulfate. In the preparation of this charge composition, the Empol® 1022 oligomerized fatty acid, the dodecylbenzene sulfonic acid, the sodium salt of a condensed naphthalene sulfonic acid and the 30 percent by weight aqueous ammonium hydroxide solution were premixed with 196 parts of deionized water and allowed to react for 15 minutes before adding the other components of the charge composition. The sodium formaldehyde sulfoxylate and chelated ferrous sulfate activators were premixed in a separate vessel in four parts of deionized water before they were added to the main reaction vessel and mixed with the other components in the charge composition.

This polymerization was run in a 75.7 liter reactor with agitation by two 15.2 cm Brumagim mixers at 300 rpm's (revolutions per minute). This polymerization was run at a temperature of 10° C. This temperature was maintained for 25 hours at which time the solid content of the emulsion had reached 27.6 percent. At this point, the reaction had reached approximately 80 percent conversion and 0.1 parts of sodium nitrite was added as a shortstop. The emulsion was then degassed to remove unpolymerized butadiene monomer that was present. This degassing was accomplished by applying 50.8 cm of vacuum to the emulsion for 10 hours.

Sixty pounds (27.2 kilograms) of this latex and 625 grams of a 25 percent Agerite Geltrol (a modified high molecular weight hindered phenol antioxidant available from the R. T. Vanderbilt Company, Inc) emulsion were slowly added to a vigorously agitated solution of 600 pounds (272 kilograms) of water at 60° C., 180 grams of concentrated sulfuric acid and 150 grams of Nalco™ 108 (polyelectrolyte to coagulate the latex. A yield of 7.7 kilograms of dried rubber was recovered.).

Nalco™ 108 is a commercially available polyelectrolyte with the structural formula: ##STR2## wherein n is an integer varying from about 2 to 15. Nalco™ 108 polyelectrolyte has a molecular weight ranging from about 200 to about 2000.

Nalco 108 polyelectrolyte is prepared by reacting epichlorohydrin with dimethylamine. Minor amounts of a compound with the structural formula: ##STR3## is also believed to be present in Nalco 108. The wet rubber crumb was dewatered and oven-dried.

The clarity of the carboxylated nitrile rubber recovered was not good. It also has water swell characteristics which were not suitable for utilization as a base polymer in making photopolymer formulations for printing plates utilized in flexographic printing applications. More specifically, the carboxylated nitrile rubber made exhibited a volume swell after water aging of only 8.5 percent.

COMPARATIVE EXAMPLE 2

In this experiment, a carboxylated nitrile rubber having a bound methacrylic acid content of about 14 percent was synthesized. The procedure employed was similar to the procedure described in Example 1. However, some major modifications were made. More specifically, the level of acrylonitrile in the polymer was increased to 40 percent and the level of methacrylic acid was increased to 14 percent. Accordingly, in the procedure implemented, the amount of acrylonitrile included in the charge composition was increased to 40 parts, the level of methacrylic acid was increased to 14 parts and the level of 1,3-butadiene was reduced to 46 parts. The mixture of coagulating agents was also modified by eliminating the Nalco™ 108 polyelectrolyte. Also, the level of calcium chloride was increased from 20 phr to 26.6 phr and the level of sulfuric acid was decreased from 2.6 phr to 1.5 phr.

The carboxylated nitrile rubber made by this procedure was much better than the rubber made in Example 1 for utilization as a base polymer in making photopolymer formulations for printing plates utilized in flexographic printing applications. This is because it exhibited much better clarity and because it was much more water-sensitive. More specifically, the carboxylated nitrile rubber made in Example 1 was depicted as having fair clarity and the rubber made in this experiment was depicted as having excellent clarity. The volume swell of the polymer synthesized in Example 1 after water aging was only 8.5 percent. The rubber made in this experiment had a greatly improved volume swell after water aging of 28.9 percent. However, the Mooney ML-4 viscosity of the rubber made in this experiment was much too high (approximately 90).

EXAMPLES 3-4

In this experiment, a carboxylated nitrile rubber was synthesized utilizing the techniques of this invention. The procedure employed was similar to the procedure described in Example 2 except for the fact that 0.6 phr or 2.0 phr of citric acid was added to the latex before coagulation in Examples 3 and 4, respectively. The citric acid reduced the Mooney ML-4 viscosity of the rubber to 49 at the 0.6 phr level and further reduced the Mooney ML-4 viscosity of the rubber to 40 at the 2.0 phr level.

The citric acid proved to reduce the Mooney ML-4 viscosity of the carboxylated nitrile rubber without having an adverse effect on clarity or water swell characteristics. In fact, the clarity of the rubber samples synthesized in these experiments were depicted as being excellent. The rubber made in Example 3 showed a volume swell after water aging of 54 percent and the rubber made in Example 4 showed a volume swell after water aging of 104 percent. It should be noted that a volume swell of greater than about 40 percent is considered to be good. The volume swell determinations made in these experiments were made after the rubber samples were aged in water for 24 hours.

EXAMPLE 5

A water-washable photopolymer was prepared by mixing the following components on a two-roll steel mixing mill:

    ______________________________________                                         Formulation Components      PHF                                                ______________________________________                                         Flexomer Blend:                                                                Flexomer                                                                       Goodyear X56932 carboxylated nitrile                                                                       100                                                (make by the general procedure described in                                    Examples 3-4)                                                                  Crosslinking Agent                                                             Sartomer SR-9035 ethoxylated triacrylate                                                                    15                                                Photoinitiator                                                                 Sartomer KB-1 benzyl dimethylketal                                                                          2                                                 Compatible Swell Enhancing Component:                                          Swell Enhancing Filler                                                         Union Carbide Polyox ™ N-10 polyethylene oxide                                                           15                                                Plasticizer                                                                    Dow Polyethyleneglycol E 400                                                                                15                                                Compatibilizer                                                                 Dipropylene glycol dibenzoate                                                                               20                                                Surfactant                                                                     ICI-Tween 20                 5                                                 TOTAL                       172                                                ______________________________________                                    

The above material was mixed until homogeneous. Mixing time was about 20 to 30 minutes using a temperature-controlled mill.

After the material was mixed, a sheet was molded in a press with a suitable mold to a desired thickness such as 0.067 inch (1.70 mm), with a 5 mil (0.13 mm) polyester support on one side and a protective slip film on the other side. Upon molding, the majority of the encapsulated air is removed, yielding a clear sheet. After an initial period of time in the mold at an elevated temperature (ie, about 10 minutes at 150° F.), cooling water is circulated through the press to return the material to ambient temperatures.

This clear sheet comprised of the photopolymer composition can then be used to make a flexographic printing plate containing an image by exposing the sheet as described above.

Development or washout of the exposed sheet is performed by placing the exposed sheet in an appropriate washout unit equipped with brushes and heaters to heat the washout solution to the desired temperature. For the above water-washable photopolymer composition which has been formed into a plate of given thickness, the following washout conditions were used:

    ______________________________________                                         Washout Solution:   5% Phoenix ™                                                                solution with pH of                                                            11.0                                                       Washout Temperature:                                                                               60° C.                                              Washout Time:       30 minutes                                                 ______________________________________                                    

The photopolymer composition of Example 10 had a washout rate of 0.0017 under the washout conditions stated above.

The washout rate test is performed by taking a sheet of uncured or uncrosslinked water-processable photopolymer composition and placing it in a washout unit comprising a flat plate to which the sheet is adhered and an orbital brushing surface containing brushes with 8-mil bristles in a given solution, at a set temperature. After a predetermined time, the sample is removed from the washout unit, dried and the sample thickness is measured. The decrease in thickness in thousandths of an inch divided by the time (in minutes) defines the washout rate in thousandths of an inch per minute. Higher washout rates are preferred when coupled with acceptable engineering and use characteristics.

EXAMPLE 6

A water-washable photopolymer was prepared by mixing the following components on a two-roll steel mixing mill:

    ______________________________________                                         Formulation Components     PHF                                                 ______________________________________                                         Flexomer Blend:                                                                Flexomer                                                                       Goodyear X56932 carboxylated nitrile                                                                      100                                                 Crosslinking Agent                                                             Sartomer SR-9035 ethoxylated triacrylate                                                                   15                                                 Sartomer SR9003B propoxylated diacrylate                                                                   15                                                 Photoinitiator                                                                 Sartomer KB-1 benzyl dimethylketal                                                                         2                                                  Compatible Swell Enhancing Component:                                          Swell Enhancing Filler                                                         Union Carbide Polyox ™ N-10 polyethylene oxide                                                          15                                                 Plasticizer                                                                    Dow Polyethyleneglycol E 400                                                                               15                                                 Compatibilizer                                                                 Dipropylene glycol dibenzoate                                                                              20                                                 Surfactant                                                                     ICI-Tween 20                5                                                  Dye                                                                            Green Dye                   0.02                                               TOTAL                      187.02                                              ______________________________________                                    

The above material was mixed and evaluated as in Example 5. The exposed sheet was developed using the following washout conditions:

    ______________________________________                                         Washout Solution:   5% Phoenix ™                                                                solution with pH of                                                            10.0                                                       Washout Temperature:                                                                               50° C.                                              Washout Time:       30 minutes                                                 ______________________________________                                    

The photopolymer evaluated in this experiment had a washout rate of 0.0017 inch per minute.

EXAMPLE 7

A water-washable photopolymer was prepared by mixing the following components on a two-roll steel mixing mill:

    ______________________________________                                         Formulation Components     PHF                                                 ______________________________________                                         Flexomer Blend:                                                                Flexomer                                                                       Goodyear X56932 carboxylated nitrile                                                                      100                                                 Crosslinking Agent                                                             Sartomer SR-9038 ethoxylated triacrylate                                                                   30                                                 Photoinitiator                                                                 Sartomer KB-1 benzyl dimethylketal                                                                         2                                                  Compatible Swell Enhancing Component:                                          Swell Enhancing Filler                                                         Union Carbide Polyox ™ N-10 polyethylene oxide                                                          15                                                 Plasticizer                                                                    Union Carbide polyethyleneglycol PEG 8000                                                                  15                                                 Compatibilizer                                                                 Dipropylene glycol dibenzoate                                                                              20                                                 Surfactant                                                                     ICI-Tween 20                5                                                  TOTAL                      187.00                                              ______________________________________                                    

The above material was mixed and evaluated as in Example 5. The exposed sheet was developed using the following washout conditions:

    ______________________________________                                         Washout Solution:   5% Phoenix ™                                                                solution with pH of                                                            10.9                                                       Washout Temperature:                                                                               50° C.                                              Washout Time:       18 minutes                                                 ______________________________________                                    

The photopolymer of evaluated in this experiment had a washout rate of 0.0021 inch per minute. The photopolymer composition of this experiment is a preferred embodiment of this invention.

Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims. 

What is claimed is:
 1. A process for preparing a high clarity water-sensitive carboxylated nitrile rubber composition which comprises the steps of (1) free radical polymerizing from about 30 weight percent to about 73 weight percent 1,3-butadiene, from about 20 weight percent to about 50 weight percent acrylonitrile and from about 7 weight percent to about 20 weight percent methacrylic acid in an aqueous polymerization medium in the presence of at least one emulsifier to produce a carboxylated nitrile rubber latex; (2) adjusting the pH of the carboxylated nitrile rubber latex to above 4 by the addition of a base to produce a pH-adjusted carboxylated nitrile rubber latex; (3) distributing at least one saponified oligomerized fatty acid throughout the pH-adjusted carboxylated nitrile rubber latex to produce a saponified oligomerized fatty acid containing pH-adjusted carboxylated nitrile rubber latex; (4) coagulating the saponified oligomerized fatty acid containing pH-adjusted carboxylated nitrile rubber latex by the addition of a combination of coagulating agents which consists essentially of at least one strong inorganic acid, calcium chloride and citric acid, to separate wet carboxylated nitrile rubber composition from an aqueous phase; and (5) drying the wet carboxylated nitrile rubber composition to produce said high clarity water-sensitive carboxylated nitrile rubber composition.
 2. A process as specified in claim 1 wherein the strong inorganic acid is sulfuric acid.
 3. A process as specified in claim 1 wherein from about 0.1 phr to about 7 phr of the saponified oligomerized fatty acid is distributed throughout the pH-adjusted carboxylated nitrile rubber in step (3).
 4. A process as specified in claim 1 wherein said free radical polymerization is conducted at a temperature which is within the range of about 5° C. to about 60° C.
 5. A process as specified in claim 1 wherein from about 1 phr to about 10 phr of the citric acid is utilized in the combination of coagulating agents employed in step (4).
 6. A process as specified in claim 1 wherein from about 0.5 phr to about 3 phr of the saponified oligomerized fatty acid is distributed throughout the pH-adjusted carboxylated nitrile rubber in step (3).
 7. A process as specified in claim 1 wherein said free radical polymerization is conducted at a temperature which is within the range of about 15° C. to about 30° C.
 8. A process for preparing a high clarity water-sensitive carboxylated nitrile rubber composition which comprises the steps of (1) free radical polymerizing from about 30 weight percent to about 73 weight percent 1,3-butadiene, from about 20 weight percent to about 50 weight percent acrylonitrile and from about 7 weight percent to about 20 weight percent methacrylic acid in an aqueous polymerization medium in the presence of at least one emulsifier to produce a carboxylated nitrile rubber latex; (2) adjusting the pH of the carboxylated nitrile rubber latex to above 4 by the addition of a base to produce a pH-adjusted carboxylated nitrile rubber latex; (3) coagulating the pH-adjusted carboxylated nitrile rubber latex by the addition of a combination of coagulating agents which consists essentially of at least one strong inorganic acid, calcium chloride and citric acid, to separate wet carboxylated nitrile rubber composition from an aqueous phase; and (4) drying the wet carboxylated nitrile rubber composition to produce said high clarity water-sensitive carboxylated nitrile rubber composition.
 9. A process as specified in claim 8 wherein from 40 weight percent to 52 weight percent 1,3-butadiene, from 36 weight percent to 44 weight percent acrylonitrile and from about 12 weight percent to 16 weight percent methacrylic acid are polymerized in said aqueous polymerization medium.
 10. A process as specified in claim 8 wherein said free radical polymerization is conducted at a temperature which is within the range of about 5° C. to about 60° C.
 11. A process as specified in claim 8 wherein from about 1 phr to about 10 phr of the citric acid is utilized in the combination of coagulating agents employed in step (3).
 12. A process as specified in claim 8 wherein from 43 weight percent to 49 weight percent 1,3-butadiene, from 38 weight percent to 42 weight percent acrylonitrile and from 13 weight percent to 15 weight percent methacrylic acid are polymerized in said aqueous polymerization medium.
 13. A process as specified in claim 1 wherein from 40 weight percent to 52 weight percent 1,3-butadiene, from 36 weight percent to 44 weight percent acrylonitrile and from about 12 weight percent to 16 weight percent methacrylic acid are polymerized in said aqueous polymerization medium.
 14. A process as specified in claim 1 wherein from 43 weight percent to 49 weight percent 1,3-butadiene, from 38 weight percent to 42 weight percent acrylonitrile and from 13 weight percent to 15 weight percent methacrylic acid are polymerized in said aqueous polymerization medium.
 15. A process as specified in claim 8 wherein said free radical polymerization is conducted at a temperature which is within the range of about 15° C. to about 30° C. 